What properties make the main character of a game such that players can easily identify with it? Can the same set properties be used when designing the NPC characters players interact with, or should a different set of properties be considered?

Simply-drawn characters are more easy for the player to relate to, because they are less specifically one person, and more of a vessel for the player/reader/viewer to project themselves onto.

For the same reason as above, characters shouldn't say much - that's the Gordon Freeman effect. Most game characters don't talk, and when they do talk, on spin-off cartoons etc (e.g. Sonic), it's annoying.

In games you can express a lot about the character from the way they move and interact with the environment.

For unique costumes, raid the history books for crazy armour, uniforms, hats and dresses, and adapt to fit your setting.

Don't use anything that is too reminiscent of another famous character, e.g. blue spiky hair, red overalls etc

Well, "characters shouldn't say much" - it depends. In adventure games Characters often have their own "branded replicas" that are often repeated. For example, Guybrush Threepwood ("Yikes!", "I'm Guybrush Threepwood, a mighty pirate!"). From "Flight of the Amazon Queen": "Just my luck". Such replicas make Characters more individual.
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toprightOct 21 '10 at 23:24

You should always have an element that people can relate with. Take for example Fry from Futurama. His main purpose is to be the connection from the 20th century to the 30th century, and without him the viewer would not make much sense from the show.

This same mentality should be used with game characters, since they often live in environments that do not exist in reality.

Another example is with Mario. He is italian, a plumber and wears human like clothing. He is the only human element you see in the Super Mario world, and therefor we connect with him very well.

I'd like to second what Olafur and Iain said, but to add a comment to Iain's fourth point.

Characters can speak, and they can speak a lot, as long as you make sure that their dialouge builds the story, and that story is engaging. A good example of this in a 'male-oriented' game that still really appeals to me as a female, would be Red Dead Redemption. Totally opposite most of my points! But his story is very strong and emotionally engaging, so I still want to get behind him and support him, even if his experience is very differnt from my own.

Beyond that, I'd like to add one of the things I read in the book "Gender Inclusive Game Design" that always sticks with me:

Have a female option for the main character!

Having the option to play as male or female will already help you get your character to appeal to a wider audience, and help the female side of your audience empathize with the story.

If you can only have one character option (as is necessary in some games), try to see if you can make it female instead of male. Given the option, most women prefer to play as women, and a lot of men to prefer to play as women too! (Gives them something nice to look at. :) )

If your concept has absolutely no way it can have women in the character base, then try to make your characters a little more base appealing. Some examples would be: The characters from Team Fortress 2 (Strong silhouhettes, bright colours, good humour and many options), and Link from Legend of Zelda (Androgynous enough that let's be honest, he can be a girl if you pretend hard enough).

Character Diamond - The group of Traits (personality aspects) — usually four, but sometimes three or five—that determine how a major character sees the world, thinks, speaks, and acts. Having a Character Diamond of three to five Traits makes characters dimensional. Character Diamonds make characters interesting, but not deep—unless some of the character's Traits are also Deepening Techniques. Major NPCs, as well as the character played by the gamer, can have Character Diamonds.

A physical property of characters that can identify what type of personality and interaction they have is their head:body ratio. Typically heroes are 9-10 heads tall, regular folk are 7-8 heads tall and childish, cartoony characters are 2-3 heads tall. This is a valuable property to include when designing your characters and may help with figuring out their personal characteristics.